Steering wheel shakes at lower speeds

1999 Lexus RX300 - steering wheel shakes slightly when accelerating. Usually in the 20-45mph range. Usually not at higher speeds. Doesn't happen every time we drive. Sometimes it's more noticeable than others. Have had the tires balanced 2x in the last month or so (both times out of balance). Had the tires rotated probably in the past 6 months.
Back tires show more wear than the front, although, it appears to be even wear on all 4 tires (bought all 4 tires at the same time).
Rotate tires? Other issues? Thanks!